1. Field of the Invention
Aspects of the present invention relate to a secondary battery, and more particularly, to a cylindrical secondary battery having improved safety performance in a compression test and a collision test.
2. Description of the Related Art
Generally, secondary batteries have been actively developed for use in lightweight portable devices, such as video cameras, cellular phones, portable computers, and the like. Secondary batteries include, for example, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, and lithium secondary batteries. Lithium secondary batteries have been widely used for advanced electronic devices, because they have a high capacity, a high operating voltage, and a high energy density per unit weight. A lithium secondary battery can be classified as rectangular, cylindrical, and pouch-type, according to the structure thereof.
A cylindrical secondary battery includes: an electrode assembly formed by winding a cathode plate, an anode plate, and a separator in a jelly roll-type shape; a cylindrical can to house the electrode assembly and an electrolyte; and a cap assembly to seal the cylindrical can. A cylindrical center pin is inserted into the core of the electrode assembly, to prevent the electrode assembly from being deformed by external forces. Thus, an internal electrical short of the battery is prevented.
A compression test and a collision test are performed to evaluate the safety of cylindrical secondary batteries. The compression test determines whether a short is generated, when applying pressure to upper and lower ends of a bare cell, in a vertical direction. If the center pin of the battery is compressed by the compression test, a short can occur.
The collision test determines whether a short is generated, when applying an external impact to the middle of the bare cell. If the electrode assembly of the battery is deformed against the center pin, a short can occur.
Generally, in a cylindrical secondary battery the center pin is inserted into the core of the electrode assembly, to support the electrode assembly, and to provide an electrolyte path. Results of the compression and collision tests differ, according to the thickness of the center pin. In other words, when a center pin having a wall thickness of 0.3 mm (relatively thick) is inserted into the cylindrical secondary battery, and the collision test is performed, a short does not occur. However, in the compression test, the center pin is deformed, to cause a short in the electrode assembly. When a center pin having a wall thickness of 0.1 mm (relatively thin) is inserted, a short does not occur in the compression test, but does occur in the collision test.
As described above, a battery having a thick center pin passes the collision test, but fails the compression test, because the center pin is deformed to cause the short. On the other hand, a battery having a thin center pin passes the compression test, but fails the collision test, because the center pin is deformed to cause the short.
As described above, there is a problem that a short cannot be prevented, regardless of whether the center pin is thick or thin. Thus, it is desirable that the center pin is as thin as possible, while still providing an electrolyte path, and preventing a short. In addition, as the size of the center pin is increased, the capacity of the battery is decreased.